1. Field of the Invention
The present invention relates to a power conversion system for converting between a.c. and d.c. power. It is particularly, but not exclusively, concerned with power transmission between one a.c. system and another, where the power is transmitted in d.c. form and therefore has to be converted prior to, and after, transmission. The present invention also relates to a power transmission system incorporating such power conversion systems, to a control apparatus for such a power conversion system, and to a method of converting power using such a power conversion system.
2. Summary of the Prior Art
It is known to transmit power between two a.c. system via a d.c. power line. A power conversion system is connected between the power line and each of the two a.c. systems, to convert power from one a.c. system to d.c. power on the transmission line, and to convert the d.c. power on transmission line to a.c. power for the other a.c. system. In one known type of power converter, there is a pair of line commutated converters connected to the d.c. power line, the two line commutated converters normally being connected in series. The pair of line commutated converters are connected via a transformer system to the respective a.c. system. Normally, separate transformers are provided for each line commutated converter, but it is possible to provide a single transformer for each pair of line commutated converters if the power requirements of the transmission system are low.
The pair of line commutated converter each has thyristors, and are controlled by a suitable control circuit to achieve the desired power conversion. In an arrangement for transmitting power from one a.c. system to another via a d.c, power line, depending on the direction of power transmission, the pair of line commutated converters of one power converter are operated to form an inverter, and the pair of line commutated converters of the other power converter are operated to form a rectifier. In such arrangements, it is known for each pair of line commutated converters to operate with their phases shifted by 30.degree., to form a 12-pulse power converter when they are connected in series. This phase shifting suppresses lower harmonics, such as 5th and 7th harmonics, thereby increasing the efficiency of power transmission.
One function of the control circuit for each pair of line commutated converters is to control the line commutated converters when there is a fault in the system, with the aim of providing minimum power losses and enable the power transmission to continue. This is done by varying the advanced control angle of the line commutated converters so that the extinction angle thereof remains as close as possible to the normal value.
It has been proposed, e.g. in JP-A-51-66455, JP-A-50-138338 and JP-A-51-101827, for such extinction angle control to be on the basis of a minimum voltage measurement from the transformer or transformers connected to the pair of line commutated converters of each power conversion system. A phase control signal is derived from that minimum voltage, together with a d.c. voltage signal and a d.c. current signal from the line commutated converters, and the phase of both line commutated converters controlled in common by that phase control signal.